Photo-electric control devices have been marketed for many years for performing various functions such as, for example, counting objects which pass by an inspection station positioned adjacent to an assembly line. In one mode of operation, pulsed beams of light are transmitted by the photo-electric control and the presence of the object is indicated by return beams reflected off of the object and detected by a photodetector in the control. This mode of opeation is designated as the proximity range mode.
In a second mode of operation, a retro-reflective target is positioned so that the objects to be detected pass between the target and the photo-electric control. Such a target will redirect transmitted light, in a direction parallel to such transmitted light. The light rebounded from the retro-reflector is sensed by the photo-electric control. In contrast with operation in the proximity mode, the retro-reflective mode will cause light to be detected upon the absence of an object. Since rebounded light is detected within a relatively wide angle in the proximity mode, and within a relatively narrow angle in the retro-reflective mode, separate photo-electric controls have been provided in the marketplace for many years to accommodate the two modes of operation. The provision of separate apparatus for this purpose is wasteful, and it is thus an object of my invention to provide a method and apparatus for facilitating operation of basically the same photoelectric control device in both modes.
It is a further object of one embodiment of my invention to eliminate potential alignment problems with respect to the positioning of the photodetector relative to the optical axis of the light detector portion of the control device.